1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer, which uses an electrophotographic printing method or an electrostatic recording method.
2. Description of the Related Art
One such type of conventional image forming apparatus has a known structure as illustrated in FIG. 13, for instance. FIG. 13 illustrates schematically an example of a basic structure of the conventional image forming apparatus.
In this example, a drum-shaped electrophotographic photosensitive member (hereinafter referred to as a “photosensitive drum”) 100 is used as an image bearing member. The photosensitive drum 100 includes a cylindrical base made of aluminum for instance, and a photosensitive material layer (e.g., an organic photosensitive material layer) formed on the surface of the cylindrical base. The photosensitive drum 100 is driven to rotate in the direction indicated by the arrow. Around the photosensitive drum 100, a charging roller 101, a laser beam scanning optical system E, a developing unit 102 as a developing device, a transferring roller 111 and a cleaner 109 are disposed in this order from the upstream side of the rotation direction.
The charging roller 101 charges the surface of the photosensitive drum 100 uniformly, and then the laser beam scanning optical system E projects an optical image so that an electrostatic latent image is formed on the photosensitive drum 100. Then, the electrostatic latent image is visualized (developed) by toner (developer) stored in the developing unit 102. The developer has a negative charging polarity. The charging polarity of the developer means the charging polarity of the developer that adheres to image regions of the latent image when the development is performed, and it has the same charging polarity as the photosensitive drum in case of reversal development. In other words, if the photosensitive drum is charged to the negative polarity in the reversal development, the charging polarity of the toner is also the negative polarity.
A recording medium, i.e., a transferring material P is fed from a sheet feeding opening (not shown) in synchronization with the above-mentioned formation of a visual image (toner image). The visual image (toner image) is transferred onto the recording medium at a substantially abutting portion between the photosensitive drum 100 and the transferring roller 111. The image on the transferring material P is fused and fixed by a fixing device 110.
The developing unit 102 includes a developing roller 103 as a developer carrying member, a supplying roller 105 for supplying nonmagnetic one-component toner (having the negative polarity) to the developing roller 103, and an agitating member 106 for conveying the toner in a container to a vicinity of the supplying roller 105. Further, the developing unit 102 includes a developer regulating blade 104 as a developer regulating member for regulating an amount of toner on the developing roller 103.
The developing roller 103 abuts the photosensitive drum 100. Therefore, the developing roller 103 is made of an elastic material. In addition, the developer regulating blade 104 utilizes a spring elasticity of a thin metal plate so as to regulate a layer thickness of the toner.
A developing bias power supply 107 applies a developing bias to the developing roller 103 so as to give a predetermined potential for moving the toner from the developing roller 103 to the photosensitive drum 100. In addition, a blade bias power supply 108 is connected to the developer regulating blade 104 and applies a blade bias so as to give a predetermined potential for stabilizing an charging amount of the toner as described in Japanese Patent Application Laid-open No. 5-11599. The blade bias power supply 108 can be one that supplies the same potential as the developing bias power supply 107, one that supplies a different potential, or other types.
The conventional method uses the layer thickness regulating blade as described above, which is press-contacted to the developing roller 103 by a predetermined pressure so that the layer thickness of the toner on the developing roller 103 can be maintained to a constant value. This developer regulating blade 104 is made of a metal that is harder than the toner, which causes a problem that the toner can be deteriorated by the developer regulating blade 104.
In order to resolve this problem, there is proposed a method involving using a conductive material like rubber or resin softer than the toner for at least a rubbing part of the layer thickness regulating blade (Japanese Patent Application Laid-Open No. H11-327291).
However, when the image forming operation was performed using the conventional structure described above, the following malfunction was found.
FIG. 14 illustrates image density with respect to desired gradation. The horizontal axis represents the gradation, which becomes more solid image toward the right side.
First, in the image forming apparatus described above as the conventional example, a voltage of the blade bias power supply 108 was set 300 volts higher than a voltage of the developing bias power supply 107 in the developer charging polarity side (e.g., in the negative side, i.e., −300 volts) to provide a potential difference between them. The gradation in this case is illustrated by the broken line G of FIG. 14. Although a desired image density was obtained, a gradation property of the image density was low. In other words, a phenomenon that the toner cannot be developed in a highlight portion (so-called low density image portion illustrated in FIG. 14 as the part (A)) (hereinafter, referred to as “white area”) occurs. In addition, a shadow portion (so-called middle density to high density portion area illustrated in FIG. 14 as the part (B)) is saturated at a maximum density soon. This gradation property is suitable for a text or other images that are desired to have a high contrast, but it is not suitable for a photographic image or other images that requires good gradation property.
If the voltage of the developing bias power supply 107 is lowered so as to suppress an amount of the developed toner, the density is decreased as a whole as illustrated by the solid line H of FIG. 14, but the gradient in the halftone portion (between the parts (A) and (B) of FIG. 14) is not changed.
In addition, it is possible to suppress the amount of the toner carried by the developing roller 103 to a certain extent even if the applied voltage of the blade bias power supply 108 is lowered. In this case, however, a phenomenon called a toner fusion occurs, in which the toner is fixed to the developer regulating blade 104.
If the conductive material of the developer regulating blade 104 is changed to a rubber having insulating property and the image output is performed in the same manner, the gradation property may be obtained but with decreased sharpness of characters on the contrary. In addition, a desired density is not obtained upon output in initial use as the developing unit.